Microstructures fabricated using silicon integrated circuit processing techniques have been developed for a wide variety of sensing and actuation applications. Compared to conventional prior art implementations in these and other applications, micro-structures provide advantages in cost, reliability, and performance. Micro-machined pressure sensors is and accelerometers are now being manufactured in quantities of over 10 million per year for a variety of uses in the medical, automotive, industrial, and instrumentation fields.
However, existing microstructures are not practical for use in certain applications due to difficulties in achieving large forces and displacements from these small devices. Some developers have circumvented this problem by attaching external, conventional actuators such as solenoids, piezoelectric elements, or pneumatic sources to microstructures to provide the needed force or displacement. These external actuators are unattractive due to their large size, critical alignment, and large power requirements. Integrated actuators, that is, microstructures where the actuator is fabricated simultaneously with the mechanical structure, are advantageous from the standpoint of cost, reliability, and ease in assembly.
Various actuation methods have been used for integrated actuators for microstructures including electrostatic, electromagnetic, thermal and thermo-pneumatic. The thermal techniques tend to provide large force but with relatively slow response times. Electromagnetic techniques are complicated by the difficulty in providing coils with a sufficient number of turns in a planar structure and the high power dissipation caused by the high currents needed to produce the desired magnetic field. Electrostatic actuation becomes attractive on a small size scale because the forces increase as the gap between elements decrease. The power dissipated by electrostatic elements tends to be low and the operating speed is usually limited only by the mechanical response of the structure.
The driving forces in prior art electrostatic actuators have been typically created using only one of two types of driving electrodes: so-called comb drive fingers or parallel plates. Parallel plate capacitors generate a force that is proportional to the square of the drive voltage and inversely proportional to the square of the gap between the plates. This behavior limits the useful range of motion for such an actuator, as at large gaps the electrostatic force is unable to overcome the restoring spring force of the actuator supports, and at gaps less than about 2/3 of the initial gap, the actuator becomes unstable when the electrostatic force overwhelms the linear restoring force. For practical microstructure elements, the useful range of motion for parallel plate actuators is less than 10 microns. Comb drive actuators, such as described in U.S. Pat. No. 5,025,346, feature a series of interdigitated electrodes whose capacitance may be used to provide a motive force that is relatively constant over a range of motion roughly equal to the length of the comb fingers, which can be made greater than 100 microns. The force available from each finger is relatively small, so that practical comb drive actuators typically have between 10 and 200 fingers to produce adequate force for a microstructure device.
A serious difficulty with prior art comb drive devices is that the maximum motion of the device is limited by so-called electromechanical side instability. In the ideal case, the side forces on each finger are exactly balanced, however if the finger is not constrained to move down the precise center of the gap, a side force will be generated by the electrodes. While the forward motive force is nearly constant with incremental deflection, the side force increases rapidly with side deflection. The instability occurs when the derivative of the side force with respect to side displacement is larger than the lateral mechanical spring constant. If this derivative exceeds the side spring constant of the motor support structure, the comb drive will snap to the side, shorting out the drive electrodes and disrupting the forward motion of the actuator. This behavior for prior art devices is described in a paper entitled "Comb-drive actuators for large displacements" by Legtenberg, Groeneveld, and Elwenspoek, in J. Micromech. Microeng. 6 (1996) pages 320-329. Their designs have a maximum displacement of about 40 microns. The design techniques in the Legtenberg paper describe the maximum displacement for conventional comb drive actuators, but do not describe designs with substantially larger deflections.
The early comb drive actuators used thin polysilicon layers provided by the so-called surface micro-machining process to fabricate the comb fingers and the moveable, laterally-driven element. This polysilicon was typically 1-2 microns thick. Since the lateral feature size of these devices was comparable to the material thickness, the stiffness of the parts to out-of-plane deflections was very low. The advent of Deep Reactive Ion Etching (DRIE) has allowed similar structures to be fabricated in single crystal silicon with typical thicknesses of 100 microns. DRIE is described in a paper entitled, "Silicon Fusion Bonding And Deep Reactive Ion Etching; A New Technology For Microstructures" by Klassen, Petersen, Noworolski, Logan, Maluf, Brown, Storment, McCully, and Kovacs, in the Proceedings Of Transducers '95 (1995), pages 556-559. These thicker structures can provide larger vertical electrode areas and substantially higher stiffnesses out of the plane of deflection. Recently, other fabrication techniques, including thicker surface micro-machined polysilicon or plated metal structures made in photolithographically defined molds have been used to increase the thickness and thus the out-of-plane stiffness of comb drive structures. None of these fabrication techniques by themselves have been used to improve the limited deflection of prior art comb drive structures.
What is needed, therefore, is an improvement in the range of deflection and characteristics of prior art comb drive actuators.
Another object of the invention is to provide an electrostatic microactuator of the above character in which side instability forces in the comb drive assembly are minimized.
Another object of the invention is to provide an electrostatic microactuator of the above character in which means for attaching or coupling the actuator to external devices is provided.
Another object of the invention is to provide an electrostatic microactuator of the above character in which the resonant characteristics of the comb drive assembly are utilized to achieve large deflections.